1. Field of the Invention
The invention relates to fastening devices and in particular to fastening devices having an expansion shell for gripping surrounding material to which an item is to be attached. More particularly, the invention relates to a bolt anchor construction for attaching metal plates to the roof of a mine tunnel and to a method of making the same from sheet metal, in which the expansion shell has a closed collar, and in which rock gripping fingers are expanded outwardly by a camming plug which is adjustably retained within the fingers by a securely connected bail.
2. Description of the Prior Art
Numerous anchor bolt constructions, fastening devices and methods of making the same have been devised for attaching articles together and in particular for attaching plates or other supports to surrounding rock, soil or strata. Examples of such anchor constructions and methods of producing them are illustrated and described in U.S. Pat. Nos. 1,139,712, 1,513,301, 1,772,064, 1,802,270, 2,667,099, 2,762,119, 2,799,201, 3,250,170, 3,3l5,,557, 3,381,567, and 3,577,825. Most of these fasteners include a bolt having an expansion-type shell mounted thereon which is expanded outwardly into gripping contact with the walls of a drilled hole by a camming-type plug.
Expansion shells for such anchors are formed of malleable iron, stamped from metal sheets or hot formed in hot rolled steel sheets. The malleable iron shells present problems of breakage upon the fingers moving outwardly to grasp the surrounding walls of a drilled hole especially where voids or noncylindrically shaped drill holes are encountered. The malleable iron shells also are expensive to produce in large quantities which are required for most mine installations. The hot rolled steel expansion shells eliminate much of the breakage problem, but also are expensive and difficult to produce in vast quantities.
Recently, expansion shells have been developed for such mine anchors which are stamped and coined from metal sheets such as shown in U.S. Pat. Nos. 3,577,825 and 3,938,420. Such stamping and coining procedures result in a strong, durable and inexpensive expansion shell which can be mass produced more conveniently and economically than prior known shells. Problems, however, have been encountered with such sheet metal expansion shells during their installation, due to the shell collars being open along one end after being folded into final shape.
The finger teeth bite into the surrounding rock of the mine roof as the plug is advanced downwardly during installation. The fingers will expand outwardly with respect to each other with only a very small amount of vertical movement. However, various degrees of rock hardness, voids, etc. adjacent the drilled hole will result in one finger of the expansion shell moving vertically and/or radially with respect to one or more of the other fingers. Thus, one finger will move downwardly and/or outwardly more than another finger of the shell causing the shell to twist and to open because of the unconnected collar ends, rather than expand uniformily outwardly. This results in anchorage failure because of uneven teeth engagement with the surrounding rocks and improper camming action between the plug and shell fingers
Known solutions to these problems have been the welding together of the open collar ends or the welding of a washer to the bottom of the collar. Either of these solutions requires a completely separate manufacturing step apart from the stamping procedure which increases the cost per shell produced. Likewise, either solution requires a welding operation which requires some degree of skill to accomplish a satisfactory weld. Also, excess welding heat will weaken the surrounding metal of the collar adjacent the welded collar edges, and not enough welding heat will result in an unsatisfactory weld.
Thus, the need has existed for a bolt anchor construction and for a method of producing the same in which the expansion shell is stamped, coined, and formed from metal strips, and in which the open collar ends of the shell can be joined during the shell stamping operation without heating, welding, or additional components being required.
Problems also have been encountered with known anchor bolt constructions which use a metal bail to retain the camming plug and bolt within the shell prior to installation, since such constructions do not permit compensation for various sizes of drilled holes. Examples, of mine roof bolt anchors having a retaining bail are shown in U.S. Pat. Nos. 927,064, 2,667,099, 2,753,750, 2,787,931, 2,878,709 and 3,022,700.
A hole drilled in the roof of a mine for insertion of an expansion bolt assembly therein may vary several sixteenths of an inch in diameter depending upon the age and condition of the drill bit, and the type of rock encountered during drilling. Most known anchor bolt constructions using a retaining bail do not permit the open finger ends of the shell to be adjusted for the varying hole sizes, in that the bail retains the camming plug in fixed rigid position against the open finger ends. Likewise, in those anchor bolt constructions having a retaining bail which do permit some adjustment of the retaining plug, the lower ends of the bail are not secured rigidly to the shell which results in additional problems during assembly, shipment, stamping and use.
Thus, the need also has existed for an anchor bolt construction having a camming plug retaining bail which is firmly attached to the shell, yet in which the bail enables limited movement of the camming plug when the anchor bolt is installed to compensate for minor variations in the hole size.